New Terpyridine-Containing Macrocycle for the Assembly of Dimeric Zn(II) and Cu(II) Complexes Coupled by Bridging Hydroxide Anions and π-Stacking Interactions

The synthesis of the new terpyridine-containing macrocycle 2,5,8,11,14-pentaaza[15](6,6‘ ‘)cyclo(2,2‘:6‘,2‘ ‘)terpyridinophane (L) is reported. The ligand contains a pentaamine chain linking the 6,6‘ ‘ positions of a terpyridine
unit. A potentiometric, 1H NMR, UV−vis spectrophotometric and fluorescence emission study on the acid−base
properties of L in aqueous solutions shows that the first four protonation steps occur on the polyamine chain,
whereas the terpyridine nitrogens are involved in proton binding only at strongly acidic pH values. L can form both
mono- and dinuclear Cu(II), Zn(II), Cd(II), and Pb(II) complexes in aqueous solution. The crystal structures of the
Zn(II) and Cd(II) complexes {[ZnLH]2(μ-OH)}(ClO4)5 (6) and {[CdLH]2(μ-Br)}(ClO4)5·4H2O (7) show that two
mononuclear [MLH]3+ units are coupled by a bridging anion (OH- in 6 and Br- in 7) and π-stacking interactions
between the terpyridine moieties. A potentiometric and spectrophotometric study shows that in the case of Cu(II)
and Zn(II) the dimeric assemblies are also formed in aqueous solution containing the ligand and the metals in a
1:1 molar ratio. Protonation of the complexes or the addition of a second metal ion leads to the disruption of the
dimers due to the increased electrostatic repulsions between the two monomeric units.